Introduced check for comm-grps with Drude relative thermostat.

Can only have one entry in comm-grps, since scaling of Drude-atom
pairs is done relative to the COM velocity.  At present, this only
works with one "absolute" thermostat scaling COM velocity.  This
limitation is present in the CHARMM implementation of these
algorithms, as well.

Change-Id: I20205ac6985ab2e32e2d55415cf7212ea638f619